Generally, this kind of derailleur is so constructed that (1) the base member and movable member are provided with support portions having recesses for receiving therein ends of the linkage members respectively, (2) the connecting pins are fitted at both ends into the support portions so as to be fixed thereto, (3) the linkage members are provided at both lengthwise ends with through-bores into which the connecting pins are freely inserted respectively, and (4) the linkage members are connected to the base member and movable member respectively, thereby forming a linkage mechanism, so that the linkage mechanism is deformed to allow the chain guide to move in reciprocation.
A conventional derailleur which is well-known, as disclosed in Japanese Patent Publication Gazette No. Sho 42-23,485, has connecting pins slanted with respect to a plane perpendicular to the axis of the multistage sprocket assembly respectively, so that when the linkage mechanism is deformed, the chain guide is moved not only axially of the multistage sprocket assembly but also radially thereof.
However, in such conventional derailleur construction, the linkage members B and C, as shown in FIG. 6, are equal in width to each other and the upper surfaces and lower surfaces of the linkage members B and C are coincident with a phantom line Y connecting the upper surfaces of the linkage members B and C and perpendicular to the axes of connecting pins D and E and a phantom line Z connecting the lower surfaces of the same and perpendicular to the axes of the pins D and E. Hence, there are gaps between the inner peripheries of the through bores of the linkage members B and C and the outer peripheries of the connecting pins fitted into the through bores, thereby creating large backlashes in the linkage mechanism.
In other words, when the chain guide is moved in reciprocation axially and radially of the multistage sprocket assembly due to deformation of the linkage mechanism, each linkage member is subjected to a torsion to create a backlash in an allowable range of the gap with respect to the base member, thereby creating a condition in which the position of the chain guide after being moved is unstable.
Therefore, in a case where a derailleur or a speed control device therefor is provided with a positioning mechanism, a difference is created between the speed change stroke and the movement of the chain guide, thereby creating a problem in that the speed change efficiency is deteriorated.
Also, when the connecting pins are slanted as described above the inner linkage member is displaced downwardly with respect to the outer linkage member, so that a height of the portion of the linkage mechanism at the base member side becomes larger, thereby creating a problem in that the portion of the linkage mechanism at the base member side is liable to strike foreign objects and also the entire derailleur of necessity is quite large.